Burned out 2 carbide end mills on lathe tool holder

samthedog

samthedog

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I had to cut down a carbide tool holder for the lathe and decided to go with milling as opposed to grinding. I put in a 20 mm carbide roughing end mill and, added cutting oil and went in fairly fast and aggressive as is the way with carbide. The thing is it just started throwing sparks and burned up the end mill. The metal work hardened instantly. I have never machined anything like it. These were quality German made end mills and they just wouldn't cut the material.

I assumed the metal would be reasonably hard, about 35 - 40 hrc and had the rpm at about 900. Didn't even scratch the material I ended up having to grind it after scrapping 2 carbide end mills. Overall a sad day.

Paul.
 
samthedog said:
I had to cut down a carbide tool holder for the lathe and decided to go with milling as opposed to grinding. I put in a 20 mm carbide roughing end mill and, added cutting oil and went in fairly fast and aggressive as is the way with carbide. The thing is it just started throwing sparks and burned up the end mill. The metal work hardened instantly. I have never machined anything like it. These were quality German made end mills and they just wouldn't cut the material.

I assumed the metal would be reasonably hard, about 35 - 40 hrc and had the rpm at about 900. Didn't even scratch the material I ended up having to grind it after scrapping 2 carbide end mills. Overall a sad day.

Paul.
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What material are you working with? Some kind of tool steel?

I had a piece of tool steel, used a dull drill bit to get through it about 3/4". I didn't know how to sharpen a drill bit then.....it work hardened instantly. I'm still sad I ruined a nice piece of tool steel :(

Doing some research, carbide should be around 70 RC.
 
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It sounds like you were turning too fast. With an expected hardness of 40RC or so I would normally start at slow RPM, maybe about 100 or less with that size endmill, with a pretty heavy feed to keep it from work hardening. There is still a possibility of chipping a carbide endmill if the setup is not very ridged.
 
Paul, not only a sad day but a expensive one as well. What brand was the insert tool holder? I recently cut down a Dorian holder. I used a 3 insert endmill holder that had some generic triangular inserts with no chip breaker. It did the job fine but a file wouldn't touch be burr on the edge of the cut. Years ago I worked for a hydraulic shop and I often had to machine induction hardened chrome shafting. We used ceramic inserts to cut through the outer.060 hard layer. It was the only thing that would touch that stuff. Problem is those inserts didn't like a interrupted cut. Scrapping inserts or endmills makes a smaller holder look cheap sometimes. I envy anyone with a surface grinder...you lucky dog.lol.

Cheers
Darrell
 
Thanks for replies gents. The mill is quite rigid however I am unsure what the steel was. It was a Chzec made tool holder. I have a tool grinder so I will sharpen the end mills again but it was a major irritation.

Jim, I believe you are right. I am keen to give it another try but am a little scared now to be honest :makingdecision:

I guess that's how people learn.

Paul.
 
Guys keep posting about burning tools up. You all need to slow down. Get information on what speeds to go. Carbide wasn't really made for cutting hardened tool steel. I get by with doing it on the lathe when I MUST. But,you were running the spindle too fast. A lathe seems to be more tolerant of turning a little hardened steel than a mill. Is there any way you can mount the holder crossways in a 4 jaw chuck,or on a faceplate and turn it down with a cheap carbide lathe tool? Run the lathe as slow as it will go.

Do you have a diamond wheel for sharpening carbide? Can you sharpen a roughing mill? You'll have to sharpen the FRONTS of the teeth if they are corncob type roughers.

Unfortunately,you won't be able to get the mill as sharp as the manufacturer since the original edges were probably polished by super fine diamond dust lapping.
 
I do have diamond wheels and dividing fixtures for sharpening the carbide. I believe that end mills I had were resharpened when I got them as they came from a prototyping company and they had a guy who sharpened the tools there.

I have certainly learned my lesson I was using this to calculate the speeds:

http://www.cuttingtoolskorea.com/english/endmill_guide.html

Paul.
 
Use an angle grinder with a cutting disk it's the cheapest way to do it.
 
Check the hardness with a file. If you can't cut it with a file you really should anneal it or you will chew up tools and grinding disks. Also I would use a Cobalt End Mill and slow down! I'm not a big fan of carbide in most situations especially on Hobby Mills.
 
A cobalt mill isn't going to cut that hard steel either. I repeat: try to arrange the hardened metal in a lathe to trim it down more cheaply with a brazed carbide cutting tool that costs a few dollars,and is easier to resharpen.
 
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